1. Field
The present disclosure relates to a backlight device and a liquid crystal display provided with the backlight device.
2. Description of the Related Art
As an apparatus for displaying an image or a video, a liquid crystal display (LCD) spreads widely. Recently, demands for enlarging a screen (liquid crystal panel) of a liquid crystal display and thinning a main body of liquid crystal display increase.
Since a liquid crystal does not emit light by itself, the liquid crystal display displays an image using an emitting device (backlight device) arranged in back of the liquid crystal panel.
As a light source used in the backlight device, a point light source such as a light emission diode (LED) or a line light source such as a cold cathode fluorescent lamp (CCFL) are cited generally.
The backlight device is classified into an edge-light type, a just-below type or the like according to a position where the light source is arranged.
The just-below type backlight device arranges light sources just in back of the liquid crystal panel at predetermined intervals. This backlight device emits apparent surface-light by allowing light emitted from the light sources to pass through a diffuser plate arranged in back of the liquid crystal panel. In this case, light intensity is relatively high in regions adjacent to the light sources and decreases with increasing distance from the light sources. If the back light device does not uniform light intensity therein, when surface-light is emitted from the light sources, a lamp image (light-dark pattern) due to the light sources appears on the screen of the liquid crystal display.
Especially, if a distance between the light sources and the diffuser plate is short to thin the backlight device, a lamp image appears on the screen of liquid crystal display remarkably. This deteriorates image quality of the liquid crystal display.
As means for eliminating a lamp image, means using a lighting curtain has been known. For example, the means is disclosed in a patent document 1 (Japanese Published Unexamined Application No. S59 (1984)-8809).
However, if a light blocking pattern of the lighting curtain is not in a right position with respect to light sources, the means can not completely eliminate a lamp image using a diffuser sheet (diffuser plate) or an optical sheet such as a prism sheet. This causes luminance on a screen of a liquid crystal display including the means to lack in uniformity.
For the means, due to deflection of the lighting curtain or thermal expansion or contraction of the lighting curtain by increase in temperature at a time when the light sources produce heat, a distance between the lighting curtain and one light source differs from one between the lighting curtain and another light source. This also causes luminance on the screen of the liquid crystal display to lack in uniformity.
When a liquid crystal display is relatively small, a diffuser plate does not bend by its own weight. However, a recent liquid crystal display is relatively large to meet a demand for enlarging a screen (liquid crystal panel) of liquid crystal display. As a liquid crystal display becomes large, a diffuser plate is also getting larger. This causes the diffuser plate to bending by its own weight.
In order to prevent the lighting curtain and/or the diffuser plate from bending by its own weight, means for using spacers to keep distances between a lighting curtain (or diffuser plate) and light sources constant has been known. For example, the means is disclosed in a patent document 2 (Japanese Published Unexamined Application No. 2003-203503). However, although preventing the lighting curtain (or diffuser plate) from bending toward a light source side of a liquid crystal display including the means, the spacer can not prevent the light curtain (or diffuser plate) from bending toward a liquid crystal panel side of the liquid crystal display. In addition, if the light curtain (or diffuser plate) bends toward the liquid crystal panel side, there is a possibility that an optical sheet attached to the diffuser plate contacts with the liquid crystal panel to damage the liquid crystal panel.
In order to prevent the lighting curtain and/or the diffuser plate from bending toward the liquid crystal panel side, a way of attaching heads of the spacers to the lighting curtain and/or the diffuser plate using an adhesive agent, a double-faced tape or the like has been proposed. In this case, each head of spacer needs to have a large surface area to get adhesive strength. If a surface area of each head of spacer is relatively large, this deteriorates visibility on a screen of a liquid crystal display. Namely, luminance on a part of the screen corresponding to attachment regions where the spacers are attached to the lighting curtain and/or the diffuser plate is lower than one on the other parts of the screen corresponding to regions other than the attachment regions.
In addition, in this way, the heads of spacers completely constrain variations in a thickness direction and a direction (in-plane direction) perpendicular to the thickness direction of the lighting curtain and/or the diffuser plate. When expansion or contraction of the lighting curtain and/or the diffuser plate by increase in temperature at a time when the light sources produce heat or usage environment occurs, the lighting curtain and/or the diffuser plate deforms (bends) in the thickness direction around the attachment regions because there are not clearances. This deformation causes a distance between the lighting curtain (or diffuser plate) and one light source to differ from one between the lighting curtain (or diffuser plate) and another light source in the regions other than the attachment regions. This deteriorates visibility on the screen of the liquid crystal display.
In order to prevent the lighting curtain and/or the diffuser plate from bending toward the liquid crystal panel side, another way of attaching fixing members standing from a back frame to the lighting curtain and/or the diffuser plate using an adhesive agent, a double-faced tape or the like has been proposed. This way is disclosed in a patent document 3 (Japanese Published Unexamined Application No. H5 (1993)-119313). However, once the fixing members are attached to the lighting curtain and/or the diffuser plate using an adhesive agent, a double-faced tape or the like, when a backlight device is taken apart in order to exchange a lamp or the like, there is a possibility that it is difficult to take apart the backlight device. Thus, the fixing members completely constrain variations in a thickness direction and a direction (in-plane direction) perpendicular to the thickness direction of the lighting curtain and/or the diffuser plate. As the above-described, when expansion or contraction of the lighting curtain and/or the diffuser plate by increase in temperature at a time when the light sources produce heat or usage environment occurs, the lighting curtain and/or the diffuser plate deforms (bends) in the thickness direction around the attachment regions because there are not clearances. This deformation causes a distance between the lighting curtain (or diffuser plate) and one light source in the attachment regions to differ from one between the lighting curtain (or diffuser plate) and another light source in the regions other than the attachment regions. This deteriorates visibility on a screen of a liquid crystal display.
In order to prevent the lighting curtain and/or the diffuser plate from bending toward the liquid crystal panel side, an alternative way of attaching the fixing members to the lighting curtain and/or the diffuser plate through transparent holes has been proposed. This way is disclosed in a patent document 4 (Japanese Patent Publication No. 4009263).
In this way, the fixing members are made of transparent material not to interfere in light to be entering the diffuser plate, and are respectively inserted into transparent holes which penetrate the semi-transparent diffuser plate.
However, if the whole fixing member is made of transparent material, when light has passed through the fixing member to reach a surface of the backlight device without being diffused in the diffuser plate, this causes light leaking. As a result, even if a part of light emitted from the light sources, which has passed through the fixing members to reach the surface of the backlight device, passes through the optical sheet, one or more bright regions (one or more regions where luminance is relatively high) appear on the liquid crystal panel due to the part of light. This causes luminance on the screen of the liquid crystal display to lack in uniformity.
Thus, it is difficult for the alternative way to keep uniformity of luminance on the screen of the liquid crystal display while preventing the diffuser plate from bending toward the liquid crystal panel side.
In order to prevent the lighting curtain and/or the diffuser plate from bending toward the liquid crystal panel side, a way of using not the spacers and the fixing members but instead ribs has been known. This way is disclosed in a patent document 5 (Japanese Published Unexamined Application No. 2003-215585). However, it is difficult to prevent the lighting curtain and/or the diffuser plate from bending toward the liquid crystal panel side using the ribs, with respect to a liquid crystal display including a large and thin liquid crystal panel more than or equal to 37 inches and a backlight device being about 10 mm thick. Even if the ribs are thickened to increase rigidity of diffuser plate, the ribs interfere in light emitted from the light sources. This causes luminance on the screen of the liquid crystal display to lack in uniformity, which deteriorates quality of the liquid crystal display.
In addition, in a conventional backlight device in which supporting members are employed to increase arrangement accuracy of the diffuse plate to prevent luminance on the diffuser plate from lacking in uniformity, there is a possibility of causing luminance on a part of the diffuser plate corresponding to engagement portions where the supporting members are engaged to the diffuser plate and adjacent portions around the engagement portions, to lack in uniformity. This problem also occurs when reinforcing members are installed to increase rigidity of the diffuse plate for preventing the diffuser plate from bending toward the liquid crystal panel side.